Efficient Thermoelectric Power Conversion from Waste Heat for Deployed Forces Project SI-1651 Chris Caylor and Rama Venkatasubramanian, Center for Solid-State Energetics, RTI International, Research Triangle Park, NC 27709; Paul Dev, D-STAR Engineering Corporation, Shelton, CT 06484; Selma Matthews, U.S. Army CERDEC, Army Power Division, Fort Belvoir, VA 22060 This work was enabled by the DARPA DTEC Program funded through the Office of Naval Research Performers Dr. Chris Caylor RTI International Specialist in thin film and bulk materials for thermoelectric applications. Dr. Rama Venkatasubramanian RTI International Director of Center for Solid-State Energetics, original work on high performance thin-film superlattice thermoelectric materials and devices Dr. Paul Dev D-STAR Engineering Corporation Project manager and president of D-Star Engineering, providing expertise in heat exchangers for thermoelectric devices with diesel engines. Ms. Selma Matthews US Army CERDEC, Army Power Division Senior Research Engineer in the CERDEC Army Power Divisions Power Technology Branch and responsible for the identification, research and development of emerging and advanced power technologies that will support future DoD platforms. Problem Statement • The DoD and the mobile forces of its branches must deal with the large carbon footprint of its operations by increasing fuel efficiency and lowering fuel use. • Fuel efficiency in mobile electric power generation is one area where improvements can be made. • Furthermore, fuel use, as well as personnel risk, could be lowered by more efficient power generation by requiring fewer fuel deliveries, which must be accomplished via manned convoys. Technical Objective: Develop waste heat recovery systems with power levels up to 500 Watts integrated with mobile diesel generators. Thermoelectric Power Generation Thermoelectric (TE) materials joined in couples (p-type and n-type) can be used for cooling or power generation Power generation uses a temperature difference (ΔT) to drive electric current. TE conversion efficiency is based on materials properties and achievable ΔT in the application (heat exchangers). ZT is TE figure-of-merit, higher ZT means more efficient power generation TE Technology for Different Temperature Ranges Bi-Polar Couple Assembled Module (BCAM) Device Technology Traditional BCAM Low stress; Scalable; Withstands 16 G’s RMS acceleration at device level RTI TE devices can be used independently or can be combined in cascades 3-4 3-4 9-10 Mid-Temp Stage 350 500 4-5 4-5 16-18 High/Mid/Low Cascade 775 800 2-3 3-4 13-14 Mid/High Cascade 650 800 4-5 4-5 13-14 Mid/Low Cascade 475 500 Incinerator / Engine Exhaust 8-11 4-5 5-6 Low-Temp Stage 150-175 200 Engine Jacket 1-2 0.5-1 2-3 Low-Temp Stage 50-75 90-100 Hot Water Specific Power (W/gram) Power Density (W/cm 2 ) Eff. (%) TE Device Structure Δ Δ ΔT (K) Avail. T hot ( o C) Field Application Scaling of Low-Temperature TE Arrays 4x4 module P ~ 1 W 6x6 module P ~ 2.1 W 16x16module 8x8 module P ~ 3.4 W Multi-Module-Arrays 1 p-n couple P ~ 0.16 W 512-couple P ~ 15 W 2048-couple P ~ 19 W Mid-Temperature Bulk Device Scale Up Progress 1 Bulk Couple 0.2 Watts 8 Bulk Couples 1.5 Watts 15 Bulk Couples 3 Watts 31 Bulk Couples 6 Watts 60 Bulk Couples 12 Watts 124 Bulk Couples 25 Watts 504 Bulk Couples 40 Watts 63 Bulk Couples Technical Approach Estimated Performance of 2-Stages vs 1-Stage Based On Measured Individual Stage Efficiencies η~5% Low-Temperature Single Stage 150 ° C 450 °C 750 ° C η~10-13% Mid-Temperature Single Stage and 2-Stage Devices η~15-18% High-Temperature 3-Stage Devices 65.8 81.0 92.4 101.9 117.6 129.7 Power/cpl (mW) T Hot ~ 500 °C 8.5 9.1 9.8 10.2 11.0 11.6 Eff (%) 7.6 56.3 6.4 39.6 125 BCA-337 6.9 47.1 5.8 33.1 150 BCA-338 8.3 66.7 6.9 46.7 100 BCA-335 8.6 73.6 7.1 51.0 75 BCA-336 9.5 88.7 8.3 67.5 50 BCA-330 10.0 97.8 8.7 73.9 25 BCA-332 Eff (%) Power/cpl (mW) Eff (%) Power/cpl (mW) T Hot ~ 450 °C T Hot ~ 400 °C T Cold (°C) Device Entrance Cap of Muffler Back of Muffler Muffler Front of Muffler DRS 3kw gen-set Generator Engine Engine Cooling Fan Hsg. DRS Fermont 3 kW Gen-set Muffler Possible location of Header Pipe Thermoelectric Device Possible Location of Thermoelectric Device Thermoelectric Device 5cm x 5cm Rough Guide to Potential Thermoelectric System Performance Phase 2 and Phase 3 will need to see increased thermal and converter efficiencies to reach 10% fuel efficiency increase go Performance for phase 1 will be based on full load operation and using single-stage thermoelectric devices CERDEC data on 3kW TQG shows that it is ~24% fuel efficient at full load (12.5kW heat input and 9.5kW waste heat, of which ½, roughly, makes it into the exhaust) Thermal efficiency = converting available waste heat to that of heat into thermoelectric device Converter efficiency = converting heat into TE device to electricity 52% 36% 14% Required Thermal Efficiency 300W 150W 50W TE Output Power Target 10% 5.0% 1.7% Fuel Efficiency Increase 12% 9.0% 7.5% TE Converter Efficiency Target Phase 3 Phase 2 Phase 1 New Q-meter System at RTI for Larger Power Device Efficiency Measurements 5-10 Watts heat flow (much larger than earlier Q-meter measurements) Used to measure single-stage and multi-stage devices for efficiency comparison Comparison of Q-meter and Calculated Heat Flows 4 5 6 7 8 9 10 11 12 13 250 350 450 550 Hot-Side Temperature (C) Thermoelectric Conversion Efficiency (%) Calculated Heat Flow Q-Stick Measured Heat Flow 11.1 2.9 8.5 25 150 500 2-stage PbTe/TAGS//SL 11.3 3.1 8.5 25 150 500 2-stage PbTe/TAGS//SL 11.6 2.9 9.0 25 130 500 2-stage PbTe/TAGS//SL 11.6 11.6 25 500 Single stage PbTe/TAGS Total η (%) Low-Temp Stage η (%) PbTe/TAGS Stage η (%) T Cold (°C) T Mid (°C) T Hot (°C) Device Single-Stage PbTe/TAGS Efficiency Matrix *SL = RTI’s Superlattice Thin Film Device 12-13% 2-stage conversion efficiency is the target for phase 2/phase 3 demonstration for SERDP Steady development of superlattice devices has shown improved performance and is poised to pass the performance of the single-stage device Entrance End Cap Muffler Characterization 0 50 100 150 200 250 300 350 400 450 500 0 0.5 1 1.5 2 2.5 3 Generator Load (kW) Temperature (°C) Cooling Air Muffler Metal Exhaust Gas